The superlative of efficient is "most efficient."
Pellets can be purchased from most any sportinggoods store or on the web from Cabelas, Bassproshop, Pyramydair, AirgunsofArizona or Dickssportinggoods.
Birds have a heart and blood vessels similar to most complex organisms. They have relatively large hearts and relatively rapid heartbeats for their size because flight requires a lot of energy to be used. Their hearts also have four (4) parts to them
They're made from coloured pellets. Most of the pellets are white - with a small proportion of coloured pellets mixed in. When the mixture is melted in the forming machine - all the pellets simply melt and mix together.
Depending on the power of the BB gun. They will kill most any small or medium size bird. However BB guns are not designed for hunting, Pellet guns and rifles make a better choice. BB do not deform on impact and the bird will suffer before death. Hunting with a pellet rifle is more humane. Pellets (especially hollow point pellets) mushroom on impact and make a clean kill.
Because of counter-current exchange, fish gills are the most efficient at gas exchange. This is followed by the continuous flow of bird lungs (due to their system of air-sacs). I don't know which animal exactly, but look to the fish for your answer.
Spur Gear is the most efficient
Usually at most hardware stores or any farm related store. If you want to high quality pellets, you can buy the pellets from the pellet factory or you can buy a wood pellet machine for making pellets for yourself at home.
The most common target air rifle pellets are made of lead, although other materials such as tin, steel alloys or even plastic are used for pellets.
More efficient and most efficient are the comparative and superlative forms of "efficient".
The most reported backyard bird is a Robin. The Robin is Americas most commen bird.
A screw is the most efficient.